https://drive.google.com/file/d/1vpg_WCdEWSc45hiH9MyXvukcMFhkGgjo/view

Above is my repair cost from a few years ago to give you an idea.

 

Thanks for sharing that. Still looking at details of the quick-estimates I have in hand, and after adding some inflation compared to yours, mine are probably reasonble

 

A Hole in the piston is very likely from running too lean.

Check the cool fuel assembly, have the injectors tested, clean out the fuel rails, look for paint chips in the fuel system between the pump and injectors.

 

A Hole in the piston is very likely from running too lean.

Check the cool fuel assembly, have the injectors tested, clean out the fuel rails, look for paint chips in the fuel system between the pump and injectors.

 

Regarding your steering pin issue, I've replaced the pins on a pair of Merc Bravo I using the JR Marine method. It is a preferred method to Merc's "Frankenstein" approach. You may want to check it out before proceeding. I'm a do it yourself guy and was able to complete this relatively easily with good results. I posted pics here on CSR with my experience. Here's JR Marine's website if you're interested.

https://www.jrmarine.com/instructions.htm

Here's my experience with JR Marine's method...

https://clubsearay.com/index.php?threads/bravo-3-steering-pin-leak.109696/

 

Great how-to write up on your repair, thanks! Looks like you did excelllent work.
25 years ago I would have tackled that work myself, using my son as a captive helper LOL... Nowadays, to spare my old bod, I write repair checks so I can take the grandkids for boat rides

 

I think once the old motor is out the problem will be obvious, 900 hrs. Could have been lean for years and you didn't know . I would think the shop would do the new engine and put the computer on it to make sure nothing is going on

 

These motors have no fuel monitoring, no oxygen sensors, no way for a computer to diagnose a lean condition. It’s a simple fuel map according to rpm, manifold vacuum and throttle position. It assumes fuel pressure and volume are correct.

It puts the pressure on the owner/mechanic to detect an issue, maybe not reaching WOT max rpm, popping, backfiring, and even removing and reading the spark plugs.

 

I thought the computer had some idea about the fuel mixture. You learn every day

 

Nope. And yes we do.

 

Thats definitely a possibilty, my original cool fuel module had the loose paint flaking issue until the engine quit running a few years ago. Tried to clean it up myself with little success. Then found out about Merc's offering despite the warranty being long gone: Got Mercruiser to cover the parts and I paid the labor to get a new CF module and injectors installed. The rail and hose were flushed and re-used.
See pic; the hole looks more melted than fractured, if that is any indication

 

Sorry to hear about the burned piston. That sucks. You may want to consider installing a A/F gauge on the replacement engine. It is an easy install and gives you a real time view of the A/F ratio.

You basically put a 1" plate between the riser and the exhaust manifold that has an O2 sensor port. Buy one of these and mount it.

 

2006 240 Sundancer, 350 Mag MPI, Bravo 3, 921 hours.

Got on plane running great for a few minutes then lost a lot of power, started running very rough and puffed a little blue/burnt oil smoke. Also lost a little motor oil into the bilge. Oil seems to have come from between the harmonic balancer and block or the timing chain cover.

Days later I started checking compression. Cylinder #8 was good, then #6 had zero. Found a hole in piston #6.
Didnt even bother checking the other cylinders.

My local Merc-certified shop has worked up various options for the engine, plus doing a steering pin job while its there to fix a small drip thats been getting around the pin for years. The plan for the pin fix is the Merc method to drill side holes for access then plug them.

Likely a long block replacement is the "economical" fix, but;

• what should be done to diagnose the cause of the failure so a new long block doesn't fail the same way?
• Anyone have experience with Jasper remanufactured LB's ($5200) or new LB's from Marine Power ($6900)
• What kind of bad surprises are common once they get into this much work?

Comments, advice, experience appreciated!

I had a hole blown in my piston last fall. It had to do with the fuel injectors.
I had the whole engine rebuilt.

 

From that picture it is a cast piston structural failure. Most OEM engines are not really sensitive lean conditions as far as "torching a hole" - possibly a burned valve but that's really it; the cylinder pressures and temperatures just aren't there. Pre-ignition knock is another thing though. Oil carbon and the subsequent pre-ignition will crack a piston as the pressures and forces on a piston are high. But you'd really hear that engine knocking.
Or, the piston simply gave up - it happens.
Or, a valve dropped and punched a hole in the piston.

Pull the cylinder head - if there is significantly more carbon in that cylinder then the piston probably broke from knocking or there is a mangled valve. Otherwise, it's just one of those things.

Reminds me - in the early 80's when nitrous oxide first became popular as a street power adder we purchases one of the early NOS plate systems and installed in on a 440 Mopar. First time I really put it to a test it torched right though the ring lands on three pistons. Cast pistons. Took it all apart, installed forged pistons and tried again but this time torched through the ring land on one piston. What the .......
Ended up the fuel solenoid was defective and not supplementing the NO2 with the necessary additional fuel. One of my school of hard knocks lessons.....

 

Thanks for everyone's comments and advice. I'll post a follow up about how this turns out.

 

Nitrous is awesome. Loved it on street cars . Hell yeah

 

2006 240 Sundancer, 350 Mag MPI, Bravo 3, 921 hours.

Got on plane running great for a few minutes then lost a lot of power, started running very rough and puffed a little blue/burnt oil smoke. Also lost a little motor oil into the bilge. Oil seems to have come from between the harmonic balancer and block or the timing chain cover.

Days later I started checking compression. Cylinder #8 was good, then #6 had zero. Found a hole in piston #6.
Didnt even bother checking the other cylinders.

My local Merc-certified shop has worked up various options for the engine, plus doing a steering pin job while its there to fix a small drip thats been getting around the pin for years. The plan for the pin fix is the Merc method to drill side holes for access then plug them.

Likely a long block replacement is the "economical" fix, but;

• what should be done to diagnose the cause of the failure so a new long block doesn't fail the same way?
• Anyone have experience with Jasper remanufactured LB's ($5200) or new LB's from Marine Power ($6900)
• What kind of bad surprises are common once they get into this much work?

Comments, advice, experience appreciated!

It detonated itself to death from running lean.

For the cost diff get the new longblock from Marine power. Michigan motorz also sells new GM base engines (tinned long block). I did that in 2016 and have zero issues. You hear too many issues with some of the remans especially jasper. If you get a new base engine this is the exact base engine VP, Mercruiser, and Marine Power use to build their packages.

 

To follow up;
Bought a brand new 350 MAG MPI long block from MichiganMotorz and had my local shop install it.
The folks at MichiganMotorz provided great customer service and answered all my questions promptly. The whole process was smooth and as advertised, no surprises, and the product was exactly as described and pictured on their webpage. Even the crating and shipping arrangements were flawless. I highly recommend them. Their price was very good and I even saved by not paying sales tax by ordering direct from them (legit out of state sale). It came standard with the recirc pump and a set of spark plugs, not all sellers include that, and they threw in gaskets and an oil filter as well.

As far as the Bravo III upper steering pin leak, my mechanic recommended removing the transom assembly to repair while the engine was out rather than one of the cut or drill methods to gain access. That was done successfully. New bushings, seal, pin, nuts and steering arm. A big expense, and maybe somewhat overkill, but its solid and drip free now. I was relieved to see the 18 year old transom plywood was dry, solid, no rot or degradation at all.

I went ahead and got a Hardin seawater pump, used my original brass pump's backside. Also, the 4 St St over-braided hydraulic hoses for the Bravo trim cylinders got replaced.

I never expected it to go flawlessly, and there was one glitch: I had to bring it back to the shop for a 2nd round of alignment of the engine with the Bravo shaft. I knew it was vibrating lightly/but noticeably at 2000 rpm when I first got the boat back, so I brought it back right away. The shop had to get a rear motor mount kit and some shims I think, and the new alignment went smooth and cured the vibration. Apparently this is common enough during re-powers that Merc has a service bulletin about it.

All the above plus a fuel injector test (all passed) and several hours into the new motor break-in, all is good so far.

 

Congrats, I have heard nothing but excellent reviews for Michigan Motors, glad you confirmed that.